refactor: improve visibility of TXT affecting apex domain

sites/platform/src/domains/steps/subdomains.md

@@ -19,6 +19,15 @@ You need to add a DNS record to make it clear you explicitly allow multiple proj
 
 ## Enable subdomains across multiple projects
 
+{{< note theme="warning" title="Apex domain cannot be re-added" >}}
+
+With the `TXT` record, restrictions apply on the apex domain.
+For example, you can't add the apex domain to another project until you remove the `TXT` record.
+
+You can see an error like `'{{<variable "YOUR_APEX_DOMAIN" >}}' is a root domain`.
+
+{{< /note >}}
+
 To ensure multiple projects can use subdomains of the same apex domain,
 add a specific `TXT` DNS record for your apex domain.
 
@@ -36,9 +45,6 @@ This ensures no other users can possibly add a subdomain of your domain to their
 Even if you don’t remove the record, your DNS records should prevent others from using a subdomain
 as long as you don’t use wildcards records pointing at {{% vendor/name %}}.
 
-However, if you don't remove the `TXT` record, restrictions apply on the apex domain.
-For example, you can't add the apex domain to another project until you remove the `TXT` record.
-
 ## Bypass locked domains
 
 In certain cases (such as if your domain was added manually by {{% vendor/name %}} support),